The HTW method has been known for a long time and is tried-and-tested technology whereby both particulate and liquid or pasty carbon-containing fuels are converted into synthesis gas. The fuels used are also heavy fuels with a very high ash content and also biomass-based fuels and carbon-containing waste materials. These are introduced into a fluidized bed, which is operated as a bubbling fluidized bed, and are gasified by means of oxygen, steam and CO2. In contrast to other gasification methods, the HTW method works at moderate temperatures at which the ash which occurs does not melt. This has operational benefits particularly in the case of corrosive ashes.
The addition of oxygen has to be metered very accurately, since excessive metering would lead to increased burn-out and therefore to an increase in the CO2 content in the synthesis gas, which must be avoided. Also, excessive metering would lead, in the immediate surroundings of the oxygen inlet points, to a meltdown of the ash particles, with the result that caking with the fluidized bed material may occur and would lead in turn to material adhering to the oxygen lances. Accurate, quick and fine regulation of the oxygen feed is therefore necessary because the fuels are partly fed discontinuously under pressure. This leads to especially stringent requirements to be fulfilled by the oxygen lances which are typically used for introducing the required oxygen into the fluidized bed reactor.
Corresponding oxygen lances are described, for example, in DE 34 39 404 C2 and DE 44 07 651 C1 which correspond to the hitherto existing prior art. In these, the problem of possible caking is solved in that, at the point of exit of the oxygen, steam addition is arranged in such a way as to form a steam film which envelops the emerging oxygen jet. The turbulences generated at the same time in the emerging gas jet have a very high steam content which prevents overheating of the entrained fluidized bed particles and thus considerably reduces the tendency to caking.
However, this technology presents problems at pressures above 8 to 10 bar. Before being added to the oxygen lance, the oxygen is usually preheated. For safety reasons, however, it would be preferable not to carry out heating above 180° C., since in this case equipment parts, in particular seals, which are customary in the industry are attacked. Above 200° C., there are statutory licensing restrictions in the use of material. If the preheated oxygen is introduced into the oxygen lance at 180° C. and if superheated steam is applied in an enveloping pipe, condensates are formed at a pressure of above 8 to 10 bar on the steam side of the oxygen-carrying pipe. These condensates change the flow conditions of the gas outlet to such a great extent that an enveloping steam film is no longer formed around the oxygen lance. This leads to the failure of the oxygen lances.